Optical measuring arrangements for measuring an electrical current in an electrical conductor are known which are based on the magneto-optic Faraday effect, and are therefore also designated as magneto-optic current transformers. In a magneto-optic current transformer, linearly polarized measuring light is transmitted through a Faraday sensor device which is arranged in the vicinity of the electrical conductor and includes an optically transparent material exhibiting the Faraday effect. Because of the Faraday effect, the magnetic field generated by the current causes a rotation of the plane of polarization of the measuring light by a rotational angle .rho., which is proportional to the path integral over the magnetic field along the path covered by the measuring light in the sensor device. The constant of proportionality is the Verdet constant V. The Verdet constant V is generally a function of the material and the temperature of the sensor device, as well as of the wavelength of the measuring light employed. In general, the sensor device surrounds the electrical conductor, so that the measuring light runs at least once around the electrical conductor in a virtually closed path. The rotational angle .rho. is, in this case, essentially directly proportional to the amplitude I of the current to be measured, in accordance with the relation EQU .rho.=N.multidot.V.multidot.I (1),
N being the number of revolutions of the measuring light around the electrical conductor. The Faraday rotational angle .rho. is determined polarimetrically by performing a polarization analysis of the measuring light running through the sensor device, in order to obtain a measuring signal for the electrical current.
It is known for the purpose of polarization analysis to use an analyzer to decompose the measuring light, after it has traversed the sensor device, into two linearly polarized light components L1 and L2 having planes of polarization, which are directed perpendicularly with respect to one another. A polarizing beam splitter can be used as the analyzer for this polarization analysis. Specifically, some of the types of polarizing beam splitters that can be used in this analysis include a Wollaston prism or a simple beam splitter having two downstream polarizers whose axes of polarization are rotated by .pi./2 or 90.degree. with respect to one another. Each of the two light components L1 and L2 is converted by one assigned photoelectric transducer into, in each case, an electrical intensity signal T1 or T2, which is proportional to the light intensity of the light component L1 or L2, respectively. A measuring signal EQU T=(T1-T2)/(T1+T2) (3)
which corresponds to the quotient of a difference and the sum of the two intensity signals T1 and T2, as described in PCT Application No. WO 95/10046, is formed from these two electrical signals.
Disregarding interference effects, this measuring signal T is given by EQU T=sin(2.pi.+.zeta.)=sin(2.multidot.N.multidot.V.multidot.I+.zeta.)(4),
.zeta. being an offset angle for I=0 A, which is a function of the angle between the plane of polarization of the measuring light on being coupled into the Faraday element and a distinctive intrinsic optical axis of the analyzer.
Although, according to equation (1), the Faraday measuring angle .rho. is itself a linear, and thus unique, function of the current I, according to equation (4) the measuring signal T is a unique function of the measuring angle .rho. only over an angular range of at most .pi./2 (or 90.degree.). Consequently, it is possible using these polarimetric magneto-optic current transformers to measure uniquely only those electrical currents which lie in a current measuring range (current measuring interval) MR with an interval length of EQU .vertline.MR.vertline.=.pi./(2.multidot.N.multidot.V) (5)
It is clear from equation (5) that the magnitude .vertline.MR.vertline. of the current measuring range MR of a magneto-optic current transformer can be set by the selection of materials having different Verdet constants V for the Faraday element and/or by the number N of revolutions of the measuring light around the electrical conductor. A larger current measuring range is obtained by setting the product N.multidot.V in the denominator smaller. However, such a selection of a larger current measuring range MR is inescapably attended by a reduced measuring resolution MA of the current transformer for a given display resolution. The measuring resolution MA is defined in this case as the absolute value .vertline.MS.vertline. of the measuring sensitivity MS of the current transformer. The measuring sensitivity MS corresponds to the gradient of the characteristic curve of the magneto-optic current transformer at an operating point, and in the case of two-channel evaluation, is given according to equation (4) by EQU MS=dT/dI=2.multidot.N.multidot.V.multidot.cos(2.multidot.N.multidot.V.multi dot.I+.zeta.) (6).
It is immediately evident from equation (6) that reducing the product N.multidot.V leads, in the case of both evaluation methods, to a reduction in the measuring resolution MA=.vertline.MS.vertline..
European Patent Application No. 088 419 describes a magneto-optic current transformer in which two Faraday glass rings, which are made of Faraday materials having different Verdet constants and thus each having inherently different current measuring ranges, are arranged parallel to one another about a common electrical conductor. Each Faraday glass ring is assigned a transmission unit for transmitting linearly polarized measuring light into the glass ring and a two-channel evaluation unit for calculating a respective measuring signal for each Faraday rotational angle. The two measuring signals of the two evaluation units are fed to an OR gate, which determines a maximum signal from the two measuring signals. This maximum signal is used to switch between the measuring ranges of the two glass rings. Different measuring ranges of the two glass rings can also be obtained given the same glass material for the two glass rings by employing measuring light of different wavelengths. In this context, the wavelength dependence of the Faraday rotation is utilized.
The publication entitled "Fiber Optic Current Sensor With Optical Analog Transmission", SENSOR 93 Conference Report IV Vol. 11.1, pages 137 to 144, describes a magneto-optical current transformer for protective purposes for measuring alternating currents, in which, after traversing a Faraday optical fiber, linearly polarized light is split into two partial light signals and each of these light signals is fed to an analyzer. The intrinsic axes (axes of polarisation) of the two analyzers are directed at an angle of 45.degree. or 58.degree. is relative to one another. The light intensities passed by the analyzers are not normalized until division by their constant components, which are obtained by peak value rectification. Subsequently, a product of the normalized signals is formed and this product is then differentiated. The Faraday rotational angle is obtained directly by integration. As a result, a signal is obtained which is proportional to the current and, therefore, is not subject to measuring range limitations. However, this method is comparatively costly.
European Patent No. 208 593 describes a magneto-optic current transformer in which, after traversing a Faraday optical fiber surrounding an electrical conductor, linearly polarized measuring light is split by a beam splitter into two partial light signals and each of these partial light signals is fed to an analyzer. The intrinsic axes of the two analyzers are directed at an angle of 0.degree. and 45.degree., respectively, relative to the coupling polarization of the measuring light. This produces a first, sinusoidal signal at the output of one analyzer, and a second, cosinusoidal signal at the output of the other analyzer. These two signals are, in each case, non-unique, oscillating functions of the current in the electrical conductor, which are phase-shifted with respect to one another by an angle of 90.degree.. A unique measuring signal is now composed from these two non-unique signals by comparing the sign and the absolute values of the measuring values of the first, sinusoidal signal and of the second, cosinusoidal signal. As soon as the absolute values of the sine and cosine are equal, that is to say given an integral multiple of 45.degree., a switch is made, as a function of the sign of sine and cosine, from a unique branch of the first, sinusoidal signal to a unique branch of the second, cosinusoidal signal, or vice versa. The measuring range of this known magneto-optic current transformer is thus, in principle, unlimited. However, the method is an incremental method, with the result that the operating point for current zero must be reset anew whenever there is a failure of the electronics of the current transformer.